A CTG repeat-selective chemical screen identifies microtubule inhibitors as selective modulators of toxic CUG RNA levels Kaalak Reddya,b, Jana R. Jenquina,b, Ona L. McConnella, John D. Clearya,b, Jared I. Richardsona,b, Belinda S. Pintoa, Maja C. Haerlea, Elizabeth Delgadoa,c, Lori Plancoa,b, Masayuki Nakamorid, Eric T. Wanga,c, and J. Andrew Berglunda,b,1 aCenter for NeuroGenetics, University of Florida College of Medicine, Gainesville, FL 32610; bThe RNA Institute, University at Albany, State University of New York, Albany, NY 12222; cDepartment of Molecular Genetics and Microbiology, University of Florida College of Medicine, Gainesville, FL 32610; and dDepartment of Neurology, Osaka University Graduate School of Medicine, 565-0871 Osaka, Japan Edited by Shuying Sun, Johns Hopkins University School of Medicine, Baltimore, MD, and accepted by Editorial Board Member Michael F. Summers September 7, 2019 (received for review February 4, 2019) A CTG repeat expansion in the DMPK gene is the causative mutation splice site by using antisense oligonucleotides to repress inclusion of myotonic dystrophy type 1 (DM1). Transcription of the expanded of this alternative exon restored muscle chloride channel protein CTG repeat produces toxic gain-of-function CUG RNA, leading to levels and eliminated myotonia in DM1 cells and mouse models disease symptoms. A screening platform that targets production (12). Hence, missplicing observed in DM1 is an important bio- or stability of the toxic CUG RNA in a selective manner has the marker for disease severity as well as for evaluating the efficacy of potential to provide new biological and therapeutic insights. A therapeutic interventions. DM1 HeLa cell model was generated that stably expresses a toxic Currently, there are no Food and Drug Administration (FDA)- r(CUG)480 and an analogous r(CUG)0 control from DMPK and was approved disease-targeting treatments for myotonic dystrophy. used to measure the ratio-metric level of r(CUG)480 versus r(CUG) Many preclinical therapeutic studies focus on targeting nuclear 0. This DM1 HeLa model recapitulates pathogenic hallmarks of r(CUG)EXP:MBNL aggregates using small molecules, antisense DM1, including CUG ribonuclear foci and missplicing of pre- oligonucleotides, RNA interference, and proteins such as RNA- mRNA targets of the muscleblind (MBNL) alternative splicing fac- BIOCHEMISTRY targeting Cas9 to free MBNL proteins and rescue DM1-associated tors. Repeat-selective screening using this cell line led to the un- missplicing (13–23). Several studies have targeted transcription of expected identification of multiple microtubule inhibitors as hits that selectively reduce r(CUG)480 levels and partially rescue MBNL- the expanded CTG repeat tract using small molecules and deac- tivated Cas9, providing proof of principle for blocking r(CUG)EXP dependent missplicing. These results were validated by using the – Food and Drug Administration-approved clinical microtubule in- production as a viable therapeutic strategy in DM1 (24 26). Using hibitor colchicine in DM1 mouse and primary patient cell models. the CRISPR/Cas9 system, several studies have also demonstrated The mechanism of action was found to involve selective reduced the potential for contraction or complete removal of the expanded transcription of the CTG expansion that we hypothesize to involve CTG repeat tract (27–30). the LINC (linker of nucleoskeleton and cytoskeleton) complex. The In addition to hypothesis-based and targeted approaches to iden- unanticipated identification of microtubule inhibitors as selective tify compounds and cellular targets to modulate toxic r(CUG)EXP modulators of toxic CUG RNA opens research directions for this form of muscular dystrophy and may shed light on the biology of CTG Significance repeat expansion and inform therapeutic avenues. This approach has the potential to identify modulators of expanded repeat-containing Toxic RNA molecules containing hundreds to thousands of CUG gene expression for over 30 microsatellite expansion disorders. repeats cause myotonic dystrophy type 1 (DM1). Selectively blocking expression of this toxic CUG RNA has the potential to alternative splicing | myotonic dystrophy | muscleblind | transcription alleviate all downstream pathogenic consequences to the cell. To identify new hits and cellular targets that selectively regu- yotonic dystrophy type 1 (DM1) is the most common form late toxic CUG RNA levels, we developed a cell-based screening Mof adult-onset muscular dystrophy and is caused by the assay that permits the ratio-metric measurement of toxic CUG expansion of a CTG repeat tract within the 3′ untranslated re- RNA abundance relative to an analogous control RNA. A pilot gion (3′ UTR) of the dystrophia myotonica protein kinase gene screen identified multiple microtubule inhibitors as hits that (DMPK)(1–3). Whereas unaffected individuals generally have selectively regulate toxic CUG RNA levels in multiple DM1 models. fewer than 35 CTG repeats, DM1 patients have up to several These findings open avenues of research and therapy in DM1 and thousand repeats, particularly in primarily postmitotic tissue provide proof of concept for repeat-selective screening to identify such as skeletal muscle (4). Transcription of the mutant DMPK unanticipatedhitsinothermicrosatellite expansion diseases. allele containing the CTG repeat expansion results in the syn- thesis of toxic CUG expansion RNA, r(CUG) , that can fold Author contributions: K.R., E.T.W., and J.A.B. designed research; K.R., J.R.J., J.D.C., J.I.R., EXP B.S.P., M.C.H., E.D., L.P., and M.N. performed research; O.L.M. contributed new reagents/ into distinct hairpin structures and are bound by the muscleblind analytic tools; K.R., J.D.C., J.I.R., M.N., and E.T.W. analyzed data; and K.R. and J.A.B. wrote (MBNL) family of alternative splicing factors (5, 6). Nuclear the paper. sequestration of the MBNL family of proteins into ribonuclear The authors declare no competing interest. foci by r(CUG)EXP results in the global missplicing of MBNL This article is a PNAS Direct Submission. S.S. is a guest editor invited by the Editorial Board. target pre-mRNAs directly causing symptoms of DM1. For ex- Published under the PNAS license. ample, missplicing of insulin receptor and chloride channel pre- Data deposition: The RNA-seq data have been deposited in the Sequence Read Archive mRNA leads to insulin resistance and myotonia, respectively (6– (SRA) database, https://www.ncbi.nlm.nih.gov/sra (accession no. SRP158284). 9). Studies have identified panels of commonly misspliced targets 1To whom correspondence may be addressed. Email: [email protected]. that correlate with disease severity in DM1 (10, 11). Reversal of This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. missplicing of these targets is a strong predictor of therapeutic 1073/pnas.1901893116/-/DCSupplemental. potential. For example, targeting the chloride channel exon 7a www.pnas.org/cgi/doi/10.1073/pnas.1901893116 PNAS Latest Articles | 1of10 Downloaded by guest on September 27, 2021 levels, unbiased screens expedite the discovery of new compounds and can identify unanticipated cellular targets. Small-molecule and genetic screens have been employed in the search for ther- apeutics as well as to inform interesting new biology associated with the CTG repeat expansion (13, 31–36). However, a CTG repeat-selective screen targeting r(CUG)EXP production and/or stability in cells has not been reported. Selectivity for the ex- panded CTG repeat is important for the development of effec- tive therapy, but can also reveal new biology of CTG and other repeat expansions. To address this gap in the field, we generated a DM1 HeLa cell model permitting the sensitive ratio-metric evaluation of r(CUG)480 levels relative to an r(CUG)0 con- trol, enabling the identification of small molecules or cellular targets that selectively modulate r(CUG)EXP levels. We applied this cell line to a pilot screen of the LOPAC1280 (Sigma-Aldrich) chemical library targeting diverse cellular pathways, leading to the identification of multiple microtubule inhibitors as regu- lators of toxic r(CUG)EXP levels. We validated microtubule inhibition using the FDA-approved natural compound colchi- cine in the HSALR DM1 mouse model and in DM1 patient- derived myotubes showing a selective reduction of toxic r(CUG)EXP levels and partially correcting several DM1- associated missplicing events. This assay is a repeat-selective screen targeting r(CUG)EXP production/stability in cells and Fig. 1. Establishing a DM1 HeLa cell line for repeat-selective screening. (A) has yielded FDA-approved compounds, providing biological Schematic of constructs used to generate a stable DM1 HeLa cell model for and therapeutic insight into DM1. repeat-selective screening. Constructs are identical in sequence except for the presence of the (CTG)480i, an interrupted repeat tract consisting of Results 24 modules of [(CTG)20(CTCGA)] and unique sequence (21-bp qPCR probe Generation of a HeLa DM1 Cell Line for Repeat-Selective Screening. target and 8-bp barcode). (B) ActD treatment (20 nM) overnight followed by To screen for novel compounds and cellular targets that reduce multiplex RT-qPCR [r(CUG)480 relative to r(CUG)0] to determine CTG- expanded CUG RNA levels in a selective manner, we generated selective response in HeLa (CTG)480 cell clones 1 and 19. Data are normal- ized to DMSO control treatments. Bars represent mean ± SD of 3 biological stable dual-construct HeLa cell lines. These cell lines contain an replicates. Z′ was calculated based on the differential response by C(t) values interrupted (CTG)480 repeat tract consisting of 24 modules of of (CTG)0 versus (CTG)480 to ActD following the method of Zhang et al. (41). [(CTG)20(CTCGA)] and a (CTG)0 control tract; both contain (C) Fluorescence in situ hybridization for rCUGEXP foci using a Cy3-(CAG) exons 11–15 of human DMPK (37), each with a unique qPCR 8 probe and counterstained with DAPI for nuclei.